pkg-config: set prefixed mainProgram for cross compilation

`lib.getExe pkgsCross.riscv64.buildPackages.pkg-config` should return
`<prefix>/bin/riscv64-unknown-linux-gnu-pkg-config` not `<prefix>/bin/pkg-config`

pkgs/build-support/pkg-config-wrapper/default.nix

···
   # See description in cc-wrapper.
   suffixSalt = replaceStrings ["-" "."] ["_" "_"] targetPlatform.config;
 
+  wrapperBinName = "${targetPrefix}${baseBinName}";
 in
 
 stdenv.mkDerivation {
···
 
       echo $pkg-config > $out/nix-support/orig-pkg-config
 
-      wrap ${targetPrefix}${baseBinName} ${./pkg-config-wrapper.sh} "${getBin pkg-config}/bin/${baseBinName}"
+      wrap ${wrapperBinName} ${./pkg-config-wrapper.sh} "${getBin pkg-config}/bin/${baseBinName}"
     ''
     # symlink in share for autoconf to find macros
 
···
 
   meta =
     let pkg-config_ = optionalAttrs (pkg-config != null) pkg-config; in
-    (optionalAttrs (pkg-config_ ? meta) (removeAttrs pkg-config.meta ["priority"])) //
+    (optionalAttrs (pkg-config_ ? meta) (removeAttrs pkg-config.meta ["priority" "mainProgram"])) //
     { description =
         attrByPath ["meta" "description"] "pkg-config" pkg-config_
         + " (wrapper script)";
       priority = 10;
+      mainProgram = wrapperBinName;
   };
 }